Stem cells are defined as cells that are capable of differentiation into many other differentiated cell types. Embryonic stem cells are stem cells from embryos which are capable of differentiation into most, if not all, of the differentiated cell types of a mature body. Stem cells are referred to as pluripotent, which describes this capability of differentiating into many cell types. A category of pluripotent stem cell of high interest to the research community is the human embryonic stem cell, abbreviated here as human ES cell, which is an embryonic stem cell derived from a human embryonic source. Human embryonic stem cells are of great scientific interest because they are capable of indefinite proliferation in culture and are thus capable, at least in principle, of supplying cells and tissues for replacement of failing or defective human tissue. The existence in culture of human embryonic stem cells offers the potential of unlimited amounts of human cells and tissues for use in a variety of therapeutic protocols to assist in human health. It is envisioned in the future human embryonic stem cells will be proliferated and directed to differentiate into specific lineages so as to develop differentiated cells or tissues which can be transplanted into human bodies for therapeutic purposes. Human embryonic stem cells and the differentiated cells that may be derived from them are also powerful scientific tools for the study of human cellular and developmental systems.
The basic techniques to create and culture human embryonic stem cells have been described. The previously reported techniques do work, but there are limitations and drawbacks to many of the procedures currently used to culture human embryonic stem cells. One limitation is of particular concern. Most existing human embryonic stem cell lines have been, to one degree or another, exposed directly to mouse cells or to a medium in which mouse cells have been cultured previously. The fact that some ES cells from existing cell lines were found to exhibit the sialic residue Neu5Gc, which is not normally made by human cells, received much attention in the press. The original techniques for the generation and culture of human embryonic stem cells required the use of mouse embryonic fibroblast (MEF) feeder cells as a feeder layer on which human embryonic stem cells could be cultured. The fibroblast feeder cells acts, through some as yet incompletely understood mechanism, to encourage the stem cells to remain in an undifferentiated state. Later, it was discovered that the same phenomenon could be achieved if the stem cells were exposed to “conditioned media.” Conditioned medium is a stem cell culture medium with which feeder cells, such as MEFs, had been previously been cultured. Either the feeder cells imparted some factor to the medium or removed some factor from the medium, but the result is that conditioned medium can be used to culture stem cells without differentiation. Either culture condition, the direct growth of human ES on murine feeder cells, or the use of conditioned media, raises the concern that one or more agents such as a virus could transmit from the mouse cells to the human ES cells. If one of the objectives of human embryonic stem cell cultures is to create tissues which can ultimately be transplanted into a human body, it is highly desirable that the stem cells never have been exposed to cells of another species or to media which have been used to culture cells of another species. Accordingly, defining a culture condition, which will permit the proliferation and culture of human embryonic stem cells without a fibroblast feeder layer, is of great interest in the continued development of techniques for the long term culture of human embryonic stem cells.
A characteristic trait of human embryonic stem cells in culture is that if conditions are less than ideal, the cells have a tendency to differentiate. It is easy to induce human ES cells to differentiate while it is demanding to maintain the human ES cells in undifferentiated state in culture. Most culture conditions will results in some level of unwanted differentiation, particularly around the periphery of the growing ES cell colony. While ES cells can be cultured with some degree of unwanted differentiation, the objective is to define a culture condition that permits the culture to remain as undifferentiated as possible, i.e. with as few differentiated cells as possible. We believe that we have used particularly stringent standards to define conditions that will support the indefinite culture of undifferentiated ES cell cultures.
Several medium formulations will permit human ES cells to remain undifferentiated for some time, but that state often fails to maintain itself. In particular, we define the growth of human ES cells from an initial seed culture in a culture vessel to confluence in the same culture vessel as a “passage.” We have found several medium formulations that permit the cultivation of human ES cells for one or two passages without severe differentiation, but then the cells differentiate rapidly upon subsequent passages. We have come to believe that in order for a medium to truly support the indefinite proliferation of human ES cells without differentiation, without conditioned medium or fibroblast feeder cells, the medium must be demonstrated to support culture of human ES cells in a substantially uniform and undifferentiated state for at least five passages. It is also important that the cultures remain relatively homogenous and undifferentiated throughout the culture period and retain all of the important characteristics of human ES cells.
The state of differentiation of a stem cell culture can be assessed most easily by judging the morphological characteristics of the cells. Undifferentiated stem cells have a characteristic morphology, i.e. small and compact cells with clearly defined cell borders, a morphology which can be easily seen by examination of a stem cell culture under a microscope. By contrast, cells which have differentiated appear larger and more diffuse, with indistinct borders. While some differentiated cells can, and normally do, appear at the margin of colonies of undifferentiated cells, the optimal stem cell culture is one that proliferates in the culture vessel with only minimal numbers of cells at the periphery of the culture appearing to be differentiated. With experience, one can judge the status of differentiation and health of human ES cell cultures visually with good accuracy. A biochemical marker that is used to track the status of ES cells as undifferentiated is the presence of the transcription factor Oct4, which has come to be regarded as the most reliable marker of undifferentiated status of ES cells, and which is one of the first markers lost as undifferentiated cells begin to differentiate.